Sensitive microelectromechanical (MEM) accelerometers are needed for navigation and other applications that operate with a range of acceleration from less than .+-.10.sup.-3 g (where the term "g" is refers to the force of gravity on earth, and is approximately equal to 9.8 meters-second.sup.-2) to about .+-.10 g or greater. Such sensitivity and enhanced dynamic range for MEM accelerometers necessitates a feedback control system that periodically re-centers a proof mass in the accelerometer.
An example of a feedback control system for a MEM accelerometer is described in a thesis by Mark A. Lemkin entitled Micro Accelerometer Design with Digital Feedback Control (University of California, Berkeley, 1997, available from University Microfilms). Lemkin's feedback control system is based on the generation of one of two possible states during each feedback cycle, including a "+1" state which provides a feedback voltage to urge the proof mass in one direction and a "-1" state which provides a feedback voltage to urge the proof mass in the other direction. Lemkin's feedback control system, therefore, necessarily requires that the proof mass be urged in one or the other direction during each feedback control cycle even in instances where no adjustment to the position of the proof mass is required or desirable (e.g. when the proof mass is correctly centered). This requirement that the proof mass be urged although such movement is not needed is disadvantageous since it can lead to incremental errors in navigational positioning which can accumulate over time.
The present invention provides an improvement over the two-level feedback control system of Lemkin by providing a three-level feedback control system having an "idle" state in which the position of the accelerometer proof mass is sensed and no feedback voltage is applied to re-center the proof mass during any feedback cycle wherein the proof mass is initially substantially centered (i.e. during which the acceleration of the proof mass falls below a predetermined threshold level).
An advantage of the present invention is that the accumulation of feedback errors over time are reduced, thereby improving the accuracy of the MEM accelerometer.
A further advantage of the present invention is that mechanical resonances of the proof mass are filtered out to provide a reduction in feedback errors associated with the mechanical resonances of the proof mass, thereby resulting in an improved sensitivity and stability of the feedback control system.
Another advantage of the present invention is that the MEM accelerometer and electronic feedback control circuitry can be formed on a single substrate (e.g. comprising silicon) to form a compact integrated single- or multi-axis accelerometer.
Yet another advantage is that a dual-proof-mass MEM accelerometer structure used in certain embodiments of the present invention can result in the cancellation of common-mode signals, thereby further enhancing the accuracy of the MEM accelerometer.
These and other advantages of the present invention will become evident to those skilled in the art.